The invention relates to molding compositions burning without residue for making casting patterns.
For making metallic castings, in particular in the field of dental technology, it is usual firstly to make a casting pattern from wax, to secure to the wax a prefabricated casting passage, to cover the casting pattern with a refractory embedding composition and finally to remove the wax composition by steaming out or burning the wax at temperatures between about 500.degree. C. and 700.degree. C. The negative mold obtained is filled with a liquid metal. After cooling and removing from the mold a casting is obtained which has the same shape as the casting pattern of wax. In analagous procedure casting molds can be made for moldings from castable glass or castable glass ceramic.
In view of the disadvantages of the wax modelling technique, which are in particular the danger of undesired deformations or damage of the patterns, the method now adopted is to make such casting patterns from plastic. After initially using so-called autopolymerizing plastics such as methyl methacrylate, recently there has been an increased tendency to use photopolymerizable plastic compositions burning without residue, cf. DE-OS 3,240,907.
One difficulty which occurs when using such photopolymerizable plastic compositions is that when burning out the molding composition cracks can easily form in the casting jacket. These cracks arise due to the pronounced thermal expansion of the molding composition in the heating process and the simultaneously counteracting thermal expansion of the casting jacket. In the casting operation the liquid metal, in particular gold, penetrates into the cracks so that metal flashes arise on the molded metal part which make finishing work necessary. Apart from the resulting additional work involved there is the disadvantage of the not inappreciable material losses.
The problem underlying the invention is to provide a molding composition burning without residue for making casting patterns for metal castings which during burning out from the mold does not cause any disturbances of the casting jackets, in particular any formation of cracks.